This invention relates to composite materials with highly aligned discontinuous fibers, and a method for preparing such composites.
In the weaving of composite fibers, it is common to employ a loom. Typically, the loom includes a supply of filament or fiber materials which are arranged on a beam or a creel. The creel is loaded with a plurality of spools or bobbins upon which the filament or fiber is wound, and from which the filament or fiber can be extended onto the weaving area. The creel thus supplies continuous fiber to the loom. In conventional systems, a substantially large number of bobbins or spools containing the filament can be loaded on to the creel.
Each filament is thread through a comb and an appropriate harness heddle, extended across a loom bed which forms part of the loom, and is attached to a beam, pin block or other structure, as appropriate. The beam or pin block is drilled with a plurality of holes or other receiving structures, each of which is designed to accept a filament extending from the creel. A steel pin or other suitable means may be used to wedge a filament in the hole and therefore to ensure that it remains securely fixed across the loom bed.
The plurality of filaments which extend across the loom bed are referred to as the warp. The heddle, which is one of a set of parallel cords or wires that when mounted compose the harness, is used to guide the warp filaments to facilitate weaving. Thus, a particular warp filament, or set of warp filaments, is raised or lowered over the loom bed with respect to other filaments or sets of filaments. According to conventional weaving technology, a complement of filaments or fibers is raised or lifted by two or more harnesses on the loom, creating a space with respect to another complement of fibers and in which a shuttle trailing a fiber is able to pass. Those fibers interspaced between the complements of warp filaments are known as the weft fibers, or weft, and run at angles which are typically transverse to those of the warp filaments. The weft passes between fiber groups, keeping them separate. The positions of the harnesses alternate in a continuing cycle, and between each change of position of each fiber set, a weft is introduced between the warp filaments. The repetition of this cycle and the introduction of the weft between each change repeats as necessary to create the woven fabric.
In one aspect, the present invention is directed towards the production of a composite material having highly aligned discontinuous fiber arrays. Such discontinuous fiber arrays are of importance in that their study leads to an increased understanding of basic principles behind the forming process of such fiber arrays. Additionally, highly aligned discontinuous fiber arrays have been shown to possess not only improved strength and an ease of processing, but also the ability to create fiber arrays with properties tailored for specific applications.
The invention comprises a composite material of continuous warp filaments which have been woven, the warp filaments comprising a reinforcing fiber and an associated matrix polymer, preferably a thermoplastic. The matrix polymer may be interspersed with the reinforcing fiber or the reinforcing fiber may be coated with the thermoplastic matrix polymer. The continuous filaments form a warp in a loom and are woven into a desired pattern using a weft which holds the warp filaments apart in a specific manner and pattern.
The weft may be a polymer filament which can remain in the composite and the completed product, or it may be a removable filament or other type of device such as a wire, plate, rod, etc. which is extracted from the fabric once the fabric has been stabilized as will be discussed below. With the filament warp and weft woven into a specific pattern, a preconsolidation follows whereby the polymer is treated with heat and/or pressure to stabilize the fabric. Once stabilized, at least some of the continuous filaments are cut in selected locations. Stabilization of the material ensures that it will not disintegrate or fall apart once these selected fibers have been cut. The weft, if removable, is withdrawn at this stage. If the weft is to remain in the finished product, the warp cut preferably passes through selected warp fibers, and a part of the way through the weft, leaving the warp fibers below the weft intact.
In another aspect of the invention, the warp may be comprised of a reinforcing fiber filament, either monofilament or tow, and a matrix polymer filament which may be either thermoplastic monofilament or tow. Once more, the matrix polymer filament may also be a monofilament or tow. The polymer-coated reinforcing fibers may be continuous or discontinuous, but, if discontinuous, are bonded together by the polymer to form a continuous strand.
Looms allow for control of specific fiber placement in woven fabrics with continuous warp filaments. As mentioned above, the warp fibers are strands of thread that run from many spools on the creel, through the loom bed for weaving, and finally wind around a take-up roll as part of the finished fabric. The harness/pedal set-up permits predetermined patterns in the finished fabric by lifting specific warp fibers from the array before the shuttle adds the weft fiber. The patterns produced in the woven fabric are the result of warp fibers which have varied lengths appearing on one side of the fabric.
The warp fiber composite, which is preferably comprised of a filament and associated matrix polymer material, can be chosen from a wide selection of materials depending on the pattern required, and the ultimate use of the material. Fibers may be carbon, or glass or polymer filaments such as nylon, polyethylene, polypropylene or a combination thereof. The matrix may be thermoplastic polymers-such as nylon, polyethylene, polypropylene, PEEK (poly-ether-ether-ketone) and/or PEKK (poly-ether-ketone-ketone).
The material can be made with a simple matrix and a selected xe2x80x9cL/D ratioxe2x80x9d. It is an important feature of the invention to control the xe2x80x9caspect ratioxe2x80x9d. The aspect ratio is the length of the fiber (L) divided by the diameter (D) of the fiber, also referred to L/D.
In one composite, nylon fibers may be added to polyethylene (PE). The nylon filament is used in conjunction with a polyethylene film which acts as a matrix. In one embodiment, the nylon filament is preferably about 0.51 mm in diameter, while the PE film is approximately 1.0 mil thick; it will, however, be appreciated an almost infinite number of thicknesses can be used, in a wide array of combinations.